This invention relates to apparatus for detecting flame such as, for example, a burner flame in a combustion chamber of a boiler and, more particularly, it relates to an improvement in field-limiting devices used with such apparatus.
The flame-detecting apparatus of this kind is required to be responsive to the particular flame from one certain burner, or from certain burners, to be monitored although there may be adjacent flame from at least one other burner. Limitation of the field of the flame detector, therefore, is required. A signal developed from the flame detector, which expresses typically whether or not there is a flame, is used to control the burner for safety needs, for example.
In particular there has been developed a field-limiting device for flame-detecting apparatus employing a lens member, which focuses light from a specific flame area impinging substantially perpendicularly on the lens member. The lens is mounted in one end of a cylindrical shell and focuses light rays from the flame source, impinging on the lens, onto a light-receiving member, such as the end of a fiberglass conduit, at the other end of the shell. However, light rays incident upon the lens, not arising from the flame to be monitored, tend to give false signals and reduce the reliability of the flame-detecting apparatus.
In the solution of these problems, dust particles in the ambient atmosphere and the relatively high level of heat, to which the apparatus is subject, must be considered.